Thermally forbidden disrotatory electrocyclic ring

A computational study of the mechanism of cyclohexyne insertion into a C(0)-Ca bond of cyclic ketones has been reported to proceed through a stepwise 2 + 2-cycloaddition of cyclohexyne to the enolate, followed by thermally allowed conrotatory electrocyclic ring opening, thermally forbidden disrotatory electrocyclic ring opening, or nonpericyclic C-C bond cleavage (Scheme 34)... 

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

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